The present technology relates to electronic apparatuses, control methods, and image sensors. More specifically, the present technology relates to an electronic apparatus, a control method, and an image sensor which make it possible to easily inhibit a decrease in image quality, for example.
For example, charge-coupled device (CCD) image sensors and complementary metal-oxide semiconductor (CMOS) image sensors are available as solid-state image capture devices for capturing images, and in recent years, CMOS image sensors have attracted attention in order to respond to the demands for miniaturization.
CMOS image sensors have analog-to-digital converters (ADCs), which perform analog-to-digital (AD) conversion on analog electrical signals output by pixels having photoelectric conversion elements, such as photodiodes (PD), for performing photoelectric conversion.
One example of the ADCs for the CMOS image sensors is what we call a “reference-signal-comparing-type ADC”, which has a comparator and a counter and compares electrical signals output by pixels with a predetermined reference signal to perform AD conversion of the electrical signals.
In the reference-signal-comparing-type ADC, the comparator compares the electrical signal output by each pixel with a reference signal (such as a ramp signal) having a level that changes at a certain gradient, and the counter counts time taken for the level of the reference signal to change until the level of the electrical signal and the level of the reference signal match each other, so that the electrical signal output by the pixel is subjected to AD conversion.
AD conversion involving counting time taken for the level of the reference signal to vary until the level of the electrical signal and the level of the reference signal match each other is also called integrating AD conversion.
Meanwhile, in an image sensor, correlated double sampling (CDS) is performed to determine a difference between an AD-conversion result of a reset level indicated by an electrical signal immediately after resetting each pixel and an AD-conversion result of a signal level indicated by an electrical signal corresponding to charge accumulated in the PD in the pixel after the resetting thereof and outputs a difference resulting from the CDS as a pixel value.
Through the CDS, noise that differs (varies) depending on each pixel or a circuit for each column in the image sensor is reduced, thus making it possible to enhance the image quality.
However, when the image sensor performs the CDS, for example, there are cases in which a so-called black sunspot occurs to degrade the image quality, for example, during image capturing of a high-luminance subject, such as the sun.
That is, when an image of a subject with a high luminance is captured, blooming in which the PD in the pixel is saturated immediately after resetting of the pixel and charge overflows from the PD occurs in the image sensor.
If blooming occurs, then the charge that overflowed from the PD is accumulated in a floating diffusion (FD) in the pixel, the reset level of the pixel becomes significantly lower than the reset level obtained when an image of a subject with an appropriate luminance is captured (this reset level may also be referred to as a “normal reset level” hereinafter), reaches a signal level obtained when the PD is saturated (this signal level may also be referred to as a “saturation level” hereinafter), or reaches a level that is close to the saturation level.
In addition, during image capturing of a subject with a high luminance, the signal level also reaches the saturation level or a level close to the saturation level.
Thus, both of the reset level and the signal level reach (substantially) their saturation levels during image capturing of a subject with a high luminance. Thus, when the CDS for determining a difference between the reset level and the signal level is performed, the difference resulting from the CDS has a significantly small value.
During image capturing of a subject with a high luminance, since pixel values resulting from the CDS have small values, an image in which pixels at a portion where the luminance of the subject is high appear to be “sunken” in black is obtained. The portion of the pixels which appears to be sunken in black is called a black sunspot.
As a technology for inhibiting an image-quality decline due to occurrence of such a black sunspot, for example, Japanese Unexamined Patent Application Publication No. 2012-010055 discloses a technology in which control means for controlling a count operation of a counter is provided between an output of a comparator in an ADC and an input of the counter.